A Client Server Transaction 15 213 Most network applications are based on the clientclientserver model The course that gives CMU its Zip A server process and one or more client processes Server manages some resource Server provides service by manipulating resource for clients Network Programming April 20 2004 1 Client sends request Topics Client process Programmer s view of the Internet review Sockets interface Writing clients and servers 4 Client handles response Server process 3 Server sends response Resource 2 Server handles request Note clients and servers are processes running on hosts can be the same or different hosts 15 213 S 04 2 class26 ppt A Programmer s View of the Internet 1 IP Addresses 1 Hosts are mapped to a set of 3232 bit IP addresses addresses 3232 bit IP addresses are stored in an IP address struct IP addresses are always stored in memory in network byte order big endian byte order True in general for any integer transferred in a packet header from one machine to another 128 2 203 179 In IP V6 Host addresses are 64 bit at some point in the future a transition from IP V4 to IP V6 will happen E g the port number used to identify an Internet connection 2 The set of IP addresses is mapped to a set of identifiers called Internet domain names names Internet address structure struct in addr unsigned int s addr network byte order big endian 128 2 203 179 is mapped to www cs cmu edu Handy network byte order conversion functions htonl convert long int from host to network byte order htons convert short int from host to network byte order ntohl convert long int from network to host byte order ntohs convert short int from network to host byte order 3 A process on one Internet host can communicate with a process on another Internet host over a connection connection 3 15 213 S 04 4 Page 1 15 213 S 04 2 Domain Naming System DNS 3 Internet Connections The Internet maintains a mapping between IP addresses and domain names in a huge worldwide distributed database called DNS DNS Clients and servers communicate by sending streams of bytes over connections connections Connections are pointpoint toto point fullfull duplex 2 2 way communication and reliable Conceptually programmers can view the DNS database as a collection of millions of host entry structures DNS host entry structure struct hostent char h name char h aliases int h addrtype int h length char h addr list Client socket address 128 2 194 242 51213 official domain name of host null terminated array of domain names host address type AF INET length of an address in bytes null terminated array of in addr structs Client gethostbyname query key is a DNS domain name gethostbyaddr query key is an IP address Connection socket pair 128 2 194 242 51213 208 216 181 15 80 Client host address 128 2 194 242 Functions for retrieving host entries from DNS 5 Server socket address 208 216 181 15 80 Server host address 208 216 181 15 Note 80 is a well known port associated with Web servers Note 51213 is an ephemeral port allocated by the kernel 15 213 S 04 15 213 S 04 6 Clients Server port 80 Using Ports to Identify Services Server host 128 2 194 242 Examples of client programs Client host Web browsers ftp telnet ssh How does a client find the server Service request for 128 2 194 242 80 i e the Web server Client Web server port 80 Kernel The IP address in the server socket address identifies the host more precisely an adapter on the host Echo server port 7 The well known port in the server socket address identifies the service and thus implicitly identifies the server process that performs that service Examples of well know ports Port 7 Echo server Port 22 Secure Shell ssh server daemon Port 25 Mail server Port 80 Web server 7 Client Service request for 128 2 194 242 7 i e the echo server Web server port 80 Kernel Echo server port 7 15 213 S 04 8 Page 2 15 213 S 04 Servers Server Examples Servers are longlong running processes daemons Web server port 80 Resource files compute cycles CGI programs Service retrieves files and runs CGI programs on behalf of the client Created at boot time typically by the init process process 1 Run continuously until the machine is turned off FTP server 20 21 depreciated Each server waits for requests to arrive on a wellwell known port associated with a particular service Resource files Service stores and retrieve files Port 7 echo server Port 22 ssh server Port 25 mail server Port 80 HTTP server SSH server 22 Resource terminal Service proxies a terminal on the server machine over an excrypted and authenticated connection Also supports file transfers forwarding of ports and X11 sessions A machine that runs a server process is also often referred to as a server 9 See etc services for a comprehensive list of the services available on a Linux machine Mail server 25 Resource email spool file Service stores mail messages in spool file 15 213 S 04 10 15 213 S 04 Security Issues Sockets Interface Don t run services that you do not really need Created in the early 80 s as part of the original Berkeley distribution of Unix that contained an early version of the Internet protocols Open port detection utility nmap see http www insecure org nmap http www insecure org nmap Provides a useruser level interface to the network PortPort knocking ports appear to be closed but open for a brief period if the kernel detects a specific sequence of connection attempts Underlying basis for most Internet applications Based on client server programming model 11 15 213 S 04 12 Page 3 15 213 S 04 Overview of the Sockets Interface Client Sockets Server socket What is a socket socket bind To the kernel a socket is an endpoint of communication To an application a socket is a file descriptor that lets the application read write from to the network open listenfd open clientfd Remember All Unix I O devices including networks are modeled as files listen connect Connection request rio writen The main distinction between regular file I O and socket I O is how the application opens the socket descriptors rio readlineb rio readlineb close Clients and servers communicate with each other by reading from and writing to socket descriptors accept rio writen EOF Await connection request from next client rio readlineb close 13 15 213 S 04 Socket Address Structures Echo Client Main Routine Generic socket address include csapp h For address arguments to connect bind and accept usage echoclient host port int main int argc char argv int clientfd port char
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